[calls] about: Update copyright

[Evangelos Ribeiro Tzaras <devrtz src gnome org>]

commit 373465cc2ca52fc0ebac050914b6232d67c33ca6
Author: Evangelos Ribeiro Tzaras <evangelos tzaras puri sm>
Date:   Thu Jul 15 11:04:42 2021 +0200

    about: Update copyright

 src/calls-main-window.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/calls-main-window.c b/src/calls-main-window.c
index a7cb8366..336f4cff 100644
--- a/src/calls-main-window.c
+++ b/src/calls-main-window.c
@@ -118,7 +118,7 @@ about_action (GSimpleAction *action,
   gtk_show_about_dialog (GTK_WINDOW (self),
                          "artists", artists,
                          "authors", authors,
-                         "copyright", "Copyright © 2018 Purism",
+                         "copyright", "Copyright © 2018 - 2021 Purism",
                          "documenters", documenters,
                          "license-type", GTK_LICENSE_GPL_3_0,
                          "logo-icon-name", APP_ID,